To his followers, he was a political giant who kept the red flag flying to the end.

To his detractors, he was the most dangerous man in parliament, who sought to turn Britain into an outpost of the Eastern Bloc and in the process almost destroyed the Labour Party.

And to the KGB, he was major threat to the stability of Cold War Europe and too “stupid” to be recruited as an agent.

For Tony Benn, the former Cabinet minister and Labour radical who died in March this year, was too left wing for the Soviet Union, the KGB’s top agent in London has revealed.

Oleg Gordievsky, who led Soviet intelligence operations in Britain in the 1980s before dramatically defecting to the West, said he was filled with horror at the "catastrophic" prospect of Tony Benn taking the deputy leadership of the Labour Party, fearing it would alter the delicate balance of power in Europe.

Benn’s demand for the nationalisation of industry, trade union power, unilateral disarmament and the abolition of the monarchy turned the 1981 contest against Denis Healey into a bitter and destructive battle for Labour’s soul.

While he was a critic of the USSR’s invasion of Czechoslovakia and human rights abuses, Benn visited Moscow several times as a minister, hailing the “professionalism” of the regime after dining at the Kremlin and the “brilliance” of the space programme.

In 1967, after hosting Premier Kosygin in London, he was granted a rare invitation to attend the May Day Parade in Moscow’s Red Square – a move blocked by diplomats on the grounds it could cause “great political embarrassment”. He was also frequent guest at the Soviet Embassy in London, attending a drinks party there on the night George Blake, the Communist mole in MI6, escaped from Wormwood Scrubs.

Yet, in an interview with The Telegraph, Gordievsky reveals for the first time how Benn was quietly shunned by diplomats as a potential source and his views expunged from spies’ briefings to Moscow after they concluded his observations were nothing more than “left-wing fairytales”.

Gordievsky was a colonel in the KGB and resident-designate in London from 1982 to 1985, in charge of intelligence gathering and espionage against Margaret Thatcher’s government as the Cold War reached its dangerous endgame. Since 1974 he had been working for MI6.

He has always maintained that the late Michael Foot, the Labour leader at the time and Benn’s friend and colleague, had previously worked for the Russians as an “agent of influence”, codenamed Boot. Foot always denied the claim, and successfully sued the Sunday Times for libel.

Benn, by contrast, was regarded after little use, according to Gordievsky, now 76 and living in England. In 2007, he was appointed Companion of the Most Distinguished Order of St Michael and St George (CMG) by the Queen for "services to the security of the United Kingdom''.

A two-and-a-half hour lunch in late 1983 at the Soviet embassy with Benn led Viktor Popov, the ambassador, to declare him unreliable.

At a meeting that December, Popov ordered agents from the KGB, the Ministry of Foreign Affairs and the military intelligence directorate to remove any information gleaned from Benn from dispatches to Moscow.

“He was not intelligence material,” Gordievsky said in an interview with the Daily Telegraph.

“Popov said, when going through your reports about meetings with different politicians to be sent to Moscow, be careful. Some of them are not truthful. I said, who are you talking about? He said Tony Benn.”

The view on the Left that he was an intellectual titan was not shared by the Russians, he added.

“He was absolutely unsuitable for political life, and I don’t know why the ambassador invited him to lunch; perhaps because he had the authority of being a major left-wing figure.”

Benn’s diaries confirm he did indeed lunch with Ambassador Popov and Michael Meacher, the Labour left-winger, at the Soviet Embassy in London in October 1983.

They discussed Michael Foot’s correspondence with Andropov, the Soviet leader, asking for his views on Britain abandoning all nuclear weapons ahead of that year’s General Election.

At the meeting Popov told Benn that Soviet intelligence had forewarning of the recent US invasion of Grenada, and they discussed proposals to cut the USSR’s missile stocks.

Two years earlier, Gordievsky claims he over-ruled hardline intelligence officers who wanted to support Benn in the 1981 deputy leadership contest.

By this late era, he emphasises, the KGB no interest in sparking a socialist revolution in Britain, and was focussed preserving the balance of power between the east and west. That meant doing nothing to destabilise either political party.

“The staff of the station, brought up with a very conservative, left-wing position, said gosh, we are going to have a wonderful new deputy head of the Labour Party,” said Gordievsky.

“And I was the only one who said, are you ok in the head? If he is, he will destroy the Labour Party. He will destroy the left-wing in Britain. And some intelligence officers listening to this said, ‘Yes, Oleg, you must be right. It was an excessive expression of our admiration’."

“I sent analysis to Moscow, saying the intention of some Labour supporters to elect Tony Benn would be catastrophic, and very, very dangerous. I was very much caring for the normal political balance of the country."